1. Technical Field
The present invention relates to a transformer, an electric circuit, a current detecting method, and an output control method. In particular, the present invention relates to an electric circuit, a current detecting method, an output control method, and a transformer provided with a third winding suitable for controlling an output current of a secondary winding of the transformer.
2. Related Art
An inverter circuit for a discharge lamp that, when used in an LCD television or monitor, can emit light from the discharge lamp at several points using a simple circuit configuration is sought. An example of such a circuit is a so-called floating light-emitting circuit in which the terminals of a secondary winding in a magnetic flux leakage step-up transformer are connected to a cold-cathode lamp. Japanese Patent Application Publications No. 2007-280796 and No. 2003-173884 are attempts to monitor the state on a secondary side by providing a transformer with a third winding.
In a floating light-emitting circuit, however, there is no easy way to detect the lamp current flowing through the secondary winding of the step-up transformer, and therefore a current transformer or a photo-coupler is used to detect the lamp current. This increases the cost of the circuit. Furthermore, the monitoring of the secondary side by the inventions of the above patent applications does not go beyond simply monitoring the secondary-side voltage, and the secondary-side current cannot be monitored.
When increasing the number of lighting points in a discharge lamp, as appropriate for a backlight of an LCD device, the light emitting circuits are arranged in parallel. With this configuration, a current difference occurs between adjacent discharge lamps, causing the overall brightness to be uneven. To prevent this problem, the leakage inductance of the step-up transformer is increased to lower the Q value of each resonance circuit on the secondary side of the step-up transformer, thereby decreasing the spread of the lamp currents. Decreasing the Q values of the resonance circuits, however, increases the difference between the voltage phase and the current phase as viewed from the primary side of the step-up transformer. This worsens the power factor so that there is an increase in the ineffective current flowing through the primary winding of the step-up transformer, and also increases the heat generated by the switching elements in the step-up transformer.
Since a floating configuration involves driving two discharge lamps in series, the output voltage of the light emitting circuit is increased. This increases the number of turns in the secondary winding, thereby lowering the self-resonant frequency. When the self-resonant frequency is lowered, it becomes impossible to obtain the step-up ratio of the transformer, and the power factor as viewed from the primary winding side is worsened, increasing the heat generation. If the number of turns in the secondary winding is decreased in an attempt to decrease the heat generation, the leakage inductance on the secondary winding side drops. This results in the current at each lighting point being different, causing uneven brightness. In other words, a tradeoff must be made between uniform brightness and heat generation, and a technique is being sought to eliminate the need for such a tradeoff. There is a desire to measure the current flowing through a secondary-side circuit of a transformer not limited to circuits in a discharge lamp, and this measurement may also be desired for switching power sources, DC-DC converter circuits, inverter circuits, or the like.